Technical Field
The present invention relates to a servo control apparatus that controls a shaft of, for example, a machine tool or an arm of a robot. More specifically, the invention relates to tandem control that controls one object to be controlled, by two motors.
Related Art
In a driving mechanism of, for example, a machine tool or a robot, when, for example one object to be controlled serving as a movable portion is large in size and torque (thrust in a linearly moving motor) of one motor is insufficient for driving a shaft of the movable portion, tandem control is applied. In tandem control, a command is given to two motors so that the two motors drive the one object to be controlled. In this case, each of the two motors (a rotary motor or a linear motor) drives the one object to be controlled in a rotating direction or a linearly moving direction via a gear and a coupling element.
FIG. 8 is a schematic view of a target plant to which the tandem control is applied, converted into a rotary motor shaft. Symbols I1 and I2 denote moments of inertia of a driving shaft 1 and another driving shaft 2, respectively. Each of the driving shafts includes one motor. Symbol IL denotes a moment of inertia of one object to be controlled. A first torque transmission system between the object to be controlled and the driving shaft 1 is denoted with a first spring system including rigidity K1 and viscous resistance D1. A second torque transmission system between the object to be controlled and the driving shaft 2 is denoted with a second spring system including rigidity K2 and viscous resistance D2. Note that x1, x2, and xL denote positions of the driving shaft 1, the driving shaft 2, and the object to be controlled, respectively.
FIG. 9 is a block diagram of an exemplary position control apparatus 300 in the related art. The position control apparatus 300 controls the position xL of the object to be controlled to which the tandem drive is applied, in accordance with a position command value X produced by function generation in each constant cycle from a host device (not shown). A first shaft position control unit 100a and a second shaft position control unit 100b control the driving shaft 1 and the driving shaft 2, respectively. Note that, in the following descriptions, elements having a common shaft are denoted with suffix a (first shaft) or b (second shaft) added to the assigned numeral. Operation of the position control apparatus 300 in the related art in FIG. 9 will be described below.
First, the first shaft position control unit 100a will be described. In the present conventional example, the first shaft position control unit 100a includes a feedforward configuration in order to speed up a command response. More specifically, an acceleration and deceleration processing unit 50a applies acceleration and deceleration processing having proper acceleration and jerk to the position command value X so as to output a position command value Xc to which the acceleration and deceleration processing has been applied. A differentiator 54a applies time differentiation to the position command value Xc so as to output a speed feedforward quantity VF. Furthermore, a differentiator 55a applies time differentiation to the speed feedforward quantity VF so as to output an acceleration command value AF. An amplification factor ATF of an amplifier ATF is a constant for obtaining, for a target plant 200 shown in FIG. 8, an acceleration and deceleration torque feedforward quantity IF corresponding to motor torque that generates acceleration AF.
A feedback configuration of the first shaft position control unit 100a is as follows. First, a subtractor 51a subtracts the position x1 of the driving shaft 1 detected by a position detector (not shown), as position feedback, from the position command value Xc. A position error amplifier Kp amplifies a position error that is output of the subtractor 51a. An adder 52a adds output of the position error amplifier Kp to the speed feedforward quantity VF so as to output a speed command value V1. A subtractor 53a subtracts, from the speed command value V1, a speed v1 in which the position x1 is differentiated by a differentiator 56a. A speed error amplifier Gv typically applies proportional integral amplification to a speed error that is output from the subtractor 53a. 
An adder 57a adds output of the speed error amplifier Gv and the acceleration and deceleration torque feedforward quantity τF so as to output a torque command value τ1 that is output from the first shaft position control unit 100a. Torque control (not shown) is applied to the torque command value τ1 so that the torque command value τ1 is substantially equal to motor-generated torque. This motor-generated torque is to be input torque to be added to the side of the driving shaft 1 of the target plant 200 shown in FIG. 8.
Since the second shaft position control unit 100b has substantially the same internal structure and constituent elements as the first shaft position control unit 100a, the descriptions are omitted. A torque command value τ2 that is output from the second shaft position control unit 100b is to be input torque to be added to the side of the driving shaft 2 of the target plant 200 shown in FIG. 8. In particular, when a torque transmission system between the driving shaft 1 and the driving shaft 2 and the torque transmission system between the driving shafts and the object to be controlled include members of the same specifications, the driving systems of the driving shaft 1 and the driving shaft 2 are in equilibrium. Therefore, the first shaft position control unit 100a and the second shaft position control unit 100b can have substantially the same configuration, including set values of the position error amplifier Kp, the speed error amplifier Gv, and the amplifier ATF.
Tandem control performance of the position control apparatus 300 in the related art shown in FIG. 9 will be described below. Note that target plant conditions include I1=I2=0.1 [kg·m2], IL=0.3 [kg·m2], K1=K2=50·103 [Nm/rad], D1=D2=0 [Nm/(rad/s)] selected, and preferred control conditions (Kp, Gv, ATF) for this target plant are set so that the two driving systems are in equilibrium.
FIGS. 10 and 11 are results of disturbance responses simulated by stepwise addition of a load disturbance τdis to the object IL to be controlled in FIG. 8, in order to evaluate disturbance inhibiting performance. Symbols v1, v2, and vL denote speeds of the driving shaft 1, the driving shaft 2, and the object to be controlled, respectively. Symbols a1, a2, and aL denote acceleration of the driving shaft 1, the driving shaft 2, and the object to be controlled, respectively. Symbols Diff1 (=Xc−x1), Diff2 (=Xc−x2), DiffL (=Xc−xL) denote position deviations of the driving shaft 1, the driving shaft 2, and the object to be controlled, respectively. In this case, as clearly shown in FIG. 11, a large vibration phenomenon attributed to the spring systems can be seen in the acceleration (a1, a2, aL) in a convergence process.
Next, FIG. 12 shows results obtained by simulating responses to a quadratic-function-type acceleration command value, in order to evaluate command tracking performance. As in the disturbance response, occurrence of vibration can be seen in the acceleration (a1, a2, aL) in an acceleration process. The position x1 of the driving shaft 1 and the position x2 of the driving shaft 2 are employed as the position feedback in the position control apparatus 300 in the related art shown in FIG. 9. Therefore, in the acceleration process, on the sides of the driving shafts, tracking is achieved to the position command value. However, on the side of the object to be controlled, a tracking delay is large and a large position deviation DiffL occurs.
FIG. 13 is a diagram of another exemplary configuration of the position control apparatus in the related art used in the tandem drive. There is added, to FIG. 9, a configuration that detects a speed difference between the driving shaft 1 and the driving shaft 2 and compensates torque command values so as to reduce deflection that occurs between the driving shaft 1 and the driving shaft 2. Parts different from FIG. 9 will be described below.
A subtractor 58 subtracts the speed v2 of the driving shaft 2 from the speed v1 of the driving shaft 1 so as to output a speed difference. The speed difference is amplified by an amplifier Gd and then subtracted, by a subtractor 59, from the torque command value that is output from the first shaft position control unit 100a so as to be the torque command value τ1 for the driving shaft 1 of a position control apparatus 301. Meanwhile, output of the amplifier Gd is added to the torque command value that is output from the second shaft position control unit 100b by an adder 60 so as to be the torque command value τ2 for the driving shaft 2 of the position control apparatus 301. With this configuration, the torque command values are compensated so as to reduce occurrence of the deflection.
FIG. 14 shows results of disturbance responses simulated by stepwise addition of the load disturbance τdis to the object IL to be controlled in FIG. 8 as in FIG. 11. When the results in FIG. 14 are compared with the results in FIG. 11, it can be seen that there is little difference in a vibration characteristic and no damping effect occurs. The configuration of the position control apparatus 301 in the related art shown in FIG. 13 is believed to be effective for inhibiting vibration attributed to torque interference between the driving shaft 1 and the driving shaft 2. However, like the present target plant, when torque is transmitted between each of the driving shafts and the object to be controlled via the spring system, although a compensation value is constituted by the speed difference between the driving shafts, vibration cannot be inhibited by reducing deflection in the target plant.